Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the proton exchange membrane (PEM) fuel cell that includes a so-called “membrane electrode assembly” comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane electrolyte and a cathode on the opposite face of the membrane-electrolyte. The membrane electrode assembly is sandwiched between a pair of electrically conductive fluid distribution elements which serve as current collectors for the anode and cathode. Flow fields are provided for distributing the fuel cell's gaseous reactants over surfaces of the respective anode and cathode. The electrically conductive fluid distribution elements may themselves form a part of the flow field in the form of appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e. H2 and O2) over the surfaces of the respective anode and cathode.
A fuel cell stack comprises a plurality of the membrane electrode assemblies stacked together in electrical series. The membrane electrode assemblies are separated from one another by the impermeable, electrically conductive fluid distribution elements, also known as a bipolar plates. The bipolar plate has two major surfaces, one facing the anode of one cell and the other surface facing the cathode on the next adjacent cell in the stack. The plate electrically conducts current between the adjacent cells. Contact elements at the ends of the stack contact only the end cells and are referred to as end plates.
In a PEM fuel cell environment that employs H2 and O2 (optionally air), the bipolar plates and other contact elements (e.g. end plates) are in constant contact with acidic solutions (pH 3-5) and operate at elevated temperatures on the order of 60 degrees centigrade to 100 degrees centigrade. Moreover, the cathode operates in a highly oxidizing environment while being exposed to pressurized air. The anode is constantly exposed to a harsh environment of pressurized hydrogen. Hence, many of the conventional contact elements are made from metal and must be resistant to acids, oxidation, and hydrogen embrittlement in the fuel cell environment. Metals which meet this criteria, however, are costly.
Lightweight metals such as aluminum and titanium and their alloys, as well as stainless steel, have been proposed for use in making fuel cell bipolar plates. Such metals are more conductive, and can be formed into very thin plates. Unfortunately, such lightweight metals are susceptible to corrosion in the hostile fuel cell environment, and bipolar plates made therefrom either dissolve (e.g. in the case of aluminum), or form a highly electronically resistive, passivating oxide film on their surface (e.g. in the case of titanium, stainless steel and aluminum) that increases the internal resistance of the fuel cell and reduces its performance. To address this problem, it has been proposed to coat the lightweight metal bipolar plates with a combination of layers which are both electrically conductive and corrosion resistant to thereby protect the underlying metal. See for example Li et al., RE 37,284 E, assigned to the assignee of the present invention.
These layered coatings, however, are expensive due to the thickness that needs to be deposited onto the plates in order to protect from corrosion. Another drawback is that these thick layers degrade when subjected to high stack compression pressures, thereby decreasing the corrosion resistance.
It is desirable, therefore, for a bipolar plate to be manufactured easily and inexpensively that is corrosion resistant and has a high degree of conductivity.